ballistics

Historical image of forensic ballistics test-firing

Source: Science Against Crime, Kind S 1972

The action of squeezing the trigger of a firearm initiates a sequence of events that results in the bullet or shot exiting the barrel of the weapon and travelling through the air on its way to meeting a target.

Internal ballistics refers to the effects of bullet design, weapon design, and materials on the projectile within the barrel of the weapon (i.e. what happens to a projectile before it leaves the gun barrel) (Volgas et al 2005).

The sequence starts with the firing pin striking the base cap of the cartridge of the round or shell. This causes the primer to ignite and this in turn detonates the gunpowder, producing a large volume of combustion gases that expand the sides of the cartridge case and eventually propel the projectile along the length of the barrel.

The primer is composed of nitrocellulose material, sometimes with added nitro-glycerin. In order to reduce static electricity, grains or flakes of gunpowder are coated in carbon graphite. These may flakes of powder may be deposited on the skin following close range discharge, and appear as grey particles if unburnt, or pale green/ orange/ tan when burnt (manufacturer dependent).

The combustion gases, along with soot, particles of primer material and powder (unburnt and partially burnt), and fragments of metal from the gun barrel follow the projectile, and depending upon the range of fire may be deposited on the target surface.

Flake gunpowder, for example, can travel up to 60 cm from the muzzle of the gun (i.e. the end of the barrel), whilst ball powders may travel up to 90 cm (Dana and DiMaio 2003).

Where powder particles are deposited on skin surfaces they give rise to ‘powder tattooing’ (if they are driven into the skin), or ‘powder stippling’ (if they abrade and mark the skin surface but are not driven in).

Soot soiling of wound margins is also a useful indicator of ‘range of fire’, and usually demonstrates a close range discharge. The exit of a projectile from a weapon is also accompanied by a variable amount of flame, and in close range discharges, the wound edges may be ‘seared’, and hairs singed and burnt.

The interposition of clothing may prevent the features of secondary projectile deposition and/ or searing from occurring, and the examination of clothing by forensic scientists may provide essential evidence in cases involving firearms.

The presence or absence of such ‘secondary projectile’ markings therefore enables an assessment of ‘range of fire’ to be made, and provide a useful source of ‘trace evidence’ for forensic ballistic experts (they are not easily washed off the skin surface).

The ability of a bullet to cause a wound depends upon the amount of kinetic energy it possesses, and this in turn depends upon its mass, and more importantly its velocity.

KE = W V²/2g

By virtue of this equation, a doubling of the mass of the bullet will give rise to a doubling of the bullets kinetic energy, whilst a doubling of the velocity will result in a quadrupling of the kinetic energy (massively increasing the wounding capacity).

Santucci and Chang (2004), however, indicate that many bullets do not impart all of their velocity into the victim efficiently, and other factors must also be considered when considering wounding potential, including;

impact velocity

bullet design (blunt tip bullets crush and injure tissues more than a pointed tip bullet of the same caliber, weight and velocity)

propensity of the bullet to tumble

energy release

bullet calibre

organ system, and

tendency for the bullet to bullet to lose speed (retardation)

Additionally, the axial stability of the bullet in flight affects the amount of kinetic energy possessed by the bullet by the time it hits the target. In general terms, handguns and shotguns fire projectiles at a relatively lower velocity than rifles (with military rifles firing bullets at very high velocity).

However, muzzle velocities are highly dependant upon the make of weapon and ammunition used, and so the non-specialist should only think of the velocity ranges of these groups of weapons in very broad terms.

Firearm Type

Muzzle velocity (m/s)

Revolvers

150

Pistols

300-360

Shotguns

400

Rifles

400-1500+ (e.g. AK-47 900m/s)

Summary of muzzle velocities by firearm type (Source Knight 1996)

Thus a bullet nearing the end of it’s maximum range, which is more likely to be exhibiting instability in the form of yaw or even tumbling (turning over end-to-end) will possess less kinetic energy, such that it may not even be able to traverse the thickness of the target, and fail to give rise to an exit wound.

External ballistics examines the effect of wind, velocity, drag and gravity on the projectile in flight from the barrel to the target (i.e. what happens to a projectile before it hits its target) (Volgas et al 2005).

A temporary cavity surrounds 'high velocity' bullets (travelling faster than the speed of sound or 340 m/s) as they traverse tissue, created by the excitation of molecules causing their centrifugal oscillation. They move in a centrifugal pattern outwards from the bullet track, and then pulsate with ever-decreasing amplitude until the cavity collapses on itself (within milliseconds).

Fackler (1988) however, makes the point that there has been an 'over exageration' of the effect of velocity on temporary cavity formation in relation to more recent 'high velocity' weapons (such as the M16), and that the increased destructiveness of such weapons in comparison with those used in combat previously may be explained by projectile fragmentation instead.

The area of damage created by such cavitations can be as much as 12 times the diameter of the bullet impacting the body (Dana and DiMaio 2003 and Knight 1996). The collapse of the cavity can ‘suck’ clothing and debris into the wound – complicating treatment of such injuries. Vennemann et al (2007) have also shown experimentally that skin particles and bacteria may also be displaced back into the bullet track from the exit wound region.

Tissues are stretched and torn, and there is microvascular injury. The margin of tissue around the cavity ‘zone of extravasation’ contains non-viable muscle and haemorrhage, and surgeons have traditionally taken the view that high velocity gunshot wounds must be managed by wide debridement (Greaves et al 2001).

However, Santucci and Chang (2004) consider such radical debridement to be unnecessary, in view of the fact that the clinical effect of temporary cavity formation is variable, and is grossly exaggerated. They explain that the temporary cavity formed in the body does not have the same properties of that seen in ballistic gelatin, and that passage through clothing and skin, together with the 'containment' properties of facial planes greatly reduce bullet velocity.

Although muscle, nerve and blood vessels can be damaged by temporary cavity formation, this is often limited. The amount of tissue destruction is dependant upon tissue type; brain, liver and muscle, for example, are affected more than highly elastic tissues such as lung.

All bullets result in a permanent cavity which represents that which is seen during surgical exploration, or at autopsy, and is caused by the mechanical effects of the passage of a bullet through the body i.e. crushing, lacerating and tearing of tissues along the projectile track as well as causing secondary damage to vessels. Bone fragmentation increases the size of this permanent cavity, as well as complicating the exit wound seen, and these fragments act as 'secondary projectiles' (Santucci and Chang 2004).

The damage seen along a bullet wound track is therefore due to a combination of the direct mechanical trauma ('tissue crush'), the amount of kinetic energy lost (and the rate at which it is lost) to the tissues and the effects of the temporary cavity formation ('tissue stretch').

The effects of temporary cavity formation in the skull can be devastating, as there is no room for expansion. Explosive head wounds can thus occur, particularly where high velocity weapons are involved.

The severity of a gunshot wound therefore relies upon the amount of kinetic energy given up by a bullet. Where the victim survives, longer-term complications of firearms injuries include local tissue necrosis and infection, and where higher velocity weapons were used, ischaemia and vascular damage at locations distant to the wound track may occur.

Factors affecting the release of kinetic energy by a bullet/ projectile (Source: adapted from DiMaio 1999)

Shotgun pellets are easily deformed upon impact, and impart their kinetic energy rapidly. They are therefore unlikely to traverse the body, unless a close range shot impacts a limb, for example. Other types of ammunition, such as soft nosed bullets are specifically designed to deform on impact and impart their kinetic energy to the surrounding tissues.

Terminal ballistics refers to the study of the behaviour of the projectile in tissue (I.e. what happens to a projectile when it hits its target) (Volgas et al 2005).

wound profiles (in ballistic gelatine tests)

source for all of the above: http://ammo.ar15.com/project/Self_Defense_Ammo_FAQ/index.htm (see this site for larger versions of these images, and other gellatine test images)

ballistic simulation gellatine testing

video of rifled ammunition ballistic gelatine tests

ballistics in art

Ballistics

Brooke Bond Tea card 'Police File' (1977)

Ballistic gelatine demonstration of fragmentation of a .45 Nytrillium Fragmentable JHP bullet, fired from a 9mm pistol. Note the large cavitation effect up to aproximately 4 inches of penetration into the gel, and many radiating tracks from fragments of the bullet.